Stress corrosion cracking of X-70 pipeline steels by eletropulsing treatment in near-neutral pH solution

Slow strain rate tests (SSRT) and environmental scanning electron microscopy (ESEM) were utilized to investigate stress corrosion cracking (SCC) behavior of electropulsed POSCO and Bao Steel X-70 pipeline steels specimens in XJ solution purged with 5% CO₂ + 95% N₂ at the strain rate of 2E-6/s. The results showed that the ultimate tensile strengths (UTS) were raised considerably for the electropulsed specimens. The UTS for electropulsed POSCO X-70 pipeline steel was enhanced much more compared to electropulsed Bao Steel X-70 pipeline steel. In addition, the relationship among UTS, the original grain size and high voltage in electropulsing treatment was revealed. SCC sensitivity increased with the decreasing electrochemical potential, especially at high cathodic potential.     

Analysis of the contribution of local anodic dissolution and hydrogen embrittlement to the corrosion fatigue,of steels

On the basis of the results of the investigation of cyclic corrosion crack-growth resistance with regard for the electrochemical conditions at the tip of a corrosion-fatigue crack, we present a quantitative analysis of the mechanisms of local anodic dissolution and hydrogen embrittlement for 20N2M and 15Kh2NMF steels in distilled water. This analysis is based on the evaluation of the contributions of these mechanisms to the increase in the fatigue crack growth rate in the media. The maps of this analysis are constructed as functions of the range of the stress intensity factor and the frequency of loading. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 6. pp. 23–32, November–December, 1996.     

The role of hydrogen in the process of stress corrosion cracking of pipeline steels in dilute carbonate-bicarbonate solution

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

氢渗透电流法监测海洋大气中钢材的腐蚀速率

用Devnathan-Stachurski双电解池技术,对16Mn钢表面干湿循环时氢渗透现象进行了研究,结果表明不同表面液膜下,都有氢渗透电流的存在.氢离子渗透量与试样腐蚀失重之间存在线性关系.利用此线性关系制作的实时监测氢渗透电流的传感器,用以记录实际海洋大气中氢渗透电流,并根据氢离子渗透量与腐蚀失重之间的线性关系对海洋大气中钢材的腐蚀速率进行预测.结果表明,实际海洋大气中,氢渗透电流与环境湿度存在着对应关系,环境湿度由大变小时,氢渗透电流由小变大.环境湿度交替变化,在试样表面完成干湿循环,促进了氢的渗透,实际海洋大气与摸拟海洋大气失重取得了较好的一致性.可以用氢渗透电流传感器实时监测海洋用钢在大气中的氢渗透情况及腐蚀失重情况.     

A comparison of hydrogen embrittlement and stress corrosion cracking in high-strength steels

The purpose of this study was to compare the known behavior of hydrogen embrittled highstrength steel to the characteristics of environmentally induced failure where hydrogen is continuously generated at the specimen surface. The incubation time for the initiation of slow crack growth was accelerated by prestressing for a fixed time below the lower critical limit. These results obtained on high-strength steel in a stress corrosion environment were directly comparable to behavior of hydrogenated specimens. These data along with hydrogen diffusivity measurements and the insensitivity of the incubation time and crack growth rate to specimen thickness indicated that the stress corrosion process was controlled by the distilled water-metal surface reaction.     

Critical Assessment 17: Mechanisms of hydrogen induced cracking in pipeline steels

Hydrogen induced cracking (HIC) remains a prominent issue for oil and gas exploration in challenging environments. This assessment discusses HIC in light of hydrogen transport through pipeline steel microstructures and crack initiation and propagation processes. While there has been significant research in hydrogen permeation through steel alloys, additional understanding is necessary in microstructures specific to pipeline steels. Furthermore, a standard model for crack initiation and propagation processes needs to be established; a fracture mechanics based model, which has been used by some researchers, is presented in the present paper to predict crack propagation. Advanced characterisation techniques can help elucidate mechanisms of hydrogen induced crack growth. Ultimately, linking hydrogen transport and cracking processes during HIC will enable optimised alloy and microstructure design.     

Stress corrosion cracking of structural steels immersed in hot-dip galvanizing baths

Failures during hot-dip galvanizing are occasionally, but they are important because of the high responsibility of the evolved structures. Traditionally two processes have been related with this phenomenon: liquid metal assisted cracking and hydrogen embrittlement. Hydrogen influence is discarded after the hydrogen concentration tests performed in this paper. Besides, crack characterization tests on steel CT specimens which are immersed in liquid galvanization bath with Zn, 1.1% Sn and 0.1% Bi at 450 °C are detailed. The main obtained results were three: 1. The value of the threshold stress intensity factor, K_th, which is the lowest value to produce cracking; 2. The subcritical crack propagation rate (da/dt)_II; and 3. The failure assessment diagram (FAD) of the steel for two different load conditions. Microscopy revealed intergranular propagation and the existence of an intermetallic compound, the FeSn, at the crack tips. As a final result, the authors propose the key points of a qualitative failure model which consists of a first step of accumulation of Sn and Pb in crack tips, a second step of production of FeSn compound, and a final step of FeSn cracking due to accumulated stresses.     

High-pH inclined stress corrosion cracking in Australian and Canadian gas pipeline X65 steels

High-pH stress corrosion cracking is a form of environmental degradation of gas pipeline steels. The crack path is intergranular by nature and typically perpendicular to the maximum applied (hoop) stress (i.e. perpendicular to the pipe outer surface). Some unusual instances of cracks have been observed in Canadian and Australian X65 pipes, where cracks grow away from the perpendicular for considerable distances. This paper presents a comparative study in terms of crack morphology, mechanical properties and crystallographic texture for these Australian and Canadian pipe steels. It is shown that the crack morphologies are quite similar, the main difference being the angle at which the cracks propagate into the material. This difference could be explained by the different through-wall texture and grain aspect ratio measured in the two materials. The interdependency of crack tip plasticity, crack tip electrochemistry and anisotropy in microstructural texture seems to heavily affect the resulting inclined crack path.     

Hydrogen in stress corrosion cracking of X-70 pipeline steels in near-neutral pH solutions

The effect of hydrogen on the stress corrosion cracking (SCC) for X-70A and X-70B pipeline steels in near-neutral pH environments was studied by monotonic tensile tests on slow strain rate tests (SSRT) machine and cyclic tests on SSRT machine and fatigue machine. The results showed that SCC intensity increased at high cathodic protection potential. Hydrogen charged at high pressure enhanced SCC sensitivity. Cyclic tests under elastic strain condition could not take the specimens to fracture and the hydrogen content was very low. However, when the load was raised to produce plastic deformation, the specimens failed within limited time and there was brittle fracture appeared on the fracture surface. Moreover, the hydrogen content was higher than that under elastic strain condition. The SCC mechanism of X-70A and X-70B pipeline steels in near-neutral pH environments was related to hydrogen accumulation induced by plastic strain.     

Stress corrosion cracking of stainless steels recommended for building brine recirculation pumps

In earlier works, characterization and stress corrosion cracking (SCC) of casings of brine recirculation pumps, used in desalination plants, had been investigated. These casings which were manufactured from two types of Ni resist ductile irons have been reported to show different service lives. Material selection is believed to be one of possible factors to extend the service life of these pumps. Two types of stainless steels; UNS S31603 and UNS S32750 have been recommended as substitutes to Ni resist ductile irons. In this work, mechanical, metallurgical, electrochemical and SCC tests have been conducted on as received wrought samples made of these two types of stainless steels. Results have shown considerable higher yield and tensile strengths and corrosion resistance for the UNS S32750 over the UNS S31603. The latter steel samples have shown reproduced pitting behavior illustrated by electrochemical measurements and visual observations against no signs of pitting for UNS S32750 samples. SCC tests in hot brine environment have shown considerable less resistance to SCC for UNS S31603 samples as demonstrated by sample failures after few days of stress corrosion testing. No SCC was observed for UNS S32750 samples under the same testing conditions.     

Evaluation of the resistance of corrosion-proof steels to corrosion cracking in hydrogen sulfide solutions

We study the tendency of corrosion-proof steels of the martensitic, martensite-ferritic, austenitic, austenite-martensitic, and ferrite-austenitic classes to corrosion cracking in hydrogen-sulfide-containing media. We have established that austenitic chromium-nickel-molybdenum (08Kh17N15M13T and 10Kh17N13M2T) and ferrite-austenitic (12Kh21N5T and 02Kh22N6AM3 steels are the most resistant to fracture: the threshold stresses in NACE solution (5% NaCl solution + 0.5% CH₃ COOH saturated with H₂S, 20±2°C, pH ∼ 3) are not less than 90% of the yield point. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 6, pp. 103–107, November–December, 2005.     

Characteristics of hydrogen embrittlement,stress corrosion cracking and tempered martensite embrittlement in high-strength steels

Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diffraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel.     

Stress corrosion cracking of pyrotherm reformer tube for steam-reforming hydrogen production

A section of Pyrotherm G 25/35 Nb reformer tube was rupture-failed in a steam-reforming hydrogen plant and analysed to identify the causes of failure. Examination of the internal surface of the pipe indicated signs of heterogeneous corrosion attack in localized areas near to the primary crack site. Some of these areas were associated with fissures, although they did not penetrate through the pipe wall. Measurement of the pipe wall thickness revealed that fair amounts of the material had been consumed by corrosion. Cross-sectional examination of the dissected pipe in areas showing signs of corrosion attack and fissures revealed the presence of radial macrocracks, originating from the internal surface, and numerous microcracks in the pipe interior. Most microcracks were formed along the grain boundaries of the spin-cast microstructure. Further examination of the macrocracked surfaces revealed the presence of a granular microstructure, indicative of a brittle failure mode. Based on the characteristics exhibited by the macrocracking, the rupture failure of the reformer tube is attributed to stress corrosion cracking (SCC). The SCC is believed to be produced through synergistic reactions amongst sulfur-containing derivatives in the natural gas (feedstock), hydrogen and superheated steam in the processed gases under a mechanically-stressed environment. The presence of the mechanical stress is attributed to the bending of the pipe caused by improper suspension design.     

Effect of bainitic microstructure on the susceptibility of pipeline steels to hydrogen induced cracking

Hydrogen induced cracking (HIC) of API X80 and API X100 pipeline steels have been investigated in high pH carbonate-bicarbonate environment using slow strain rate testing (SSRT) method. It has been found that while both steels are highly susceptible to HIC, and diffusible hydrogen content is higher in API X80 than in API X100, the later steel is more vulnerable than the former at high (more negative) cathodic potential. This higher susceptibility can be primarily attributed to the combined effect of (1) separation of bainitic lath boundaries due to hydrogen trapping in these locations, (2) mobile hydrogen, and (3) stress. The charging-discharging experiments followed by SSRT experiments in air suggest that, the cracks that appeared due to lath boundary separation did not cause the reduction of ductility by themselves, rather it was the diffusible hydrogen that forced these cracks to propagate and, ruptured the steel with very low percent reduction of area (%RA). Despite the fact that the mobile hydrogen content plays a key role in causing the embrittlement, the large number of cracks in API X100 steel, resulting from the bainitic lath boundary separation at high cathodic potential, superseded the effect of higher diffusible hydrogen content in API X80 steel. The general conclusion is that bainitic lath type microstructure is more vulnerable to HIC at high cathodic potential than the ferritic/granular bainitic ones. It has been also found that applying cathodic protection can lead to excessive hydrogen embrittlement in both of the abovementioned steels in high pH carbonate-bicarbonate environment and, therefore, efforts need to be invested in developing nobler (more positive corrosion potential) and better HIC resistant steels.